Aiming at the contradiction between the narrow switching frequency operating range and the wide output voltage range of the traditional LLC resonant converter, a three-switch LLC resonant converter with a variable structure type and wide output voltage range and its control method are proposed. The converter adopts three switching tubes in the primary inverter circuit, which can realise the independent or joint operation of two transformers. Four different operating modes can be realised by controlling the ON and OFF of the primary-side switching tubes and the secondary-side auxiliary switches, which shortens the operating range of the switching frequency and widens the voltage gain range, and meets the requirements of wide output voltage range applications. Since the converter always works in the weak inductive region, it can achieve zero-voltage turn-on of the primary switch and zero-current turn-off of the secondary diode over the whole range, which has good soft-switching performance. This paper analyses the working principle, voltage characteristics and parameter design of the converter. Finally, a 1kW experimental prototype is fabricated based on the operating principle of the converter, and the experimental results show that the converter meets the requirements of wide output voltage range applications in all aspects.